An electronic delay igniter, as a substitute for a prior art chemical reaction-type igniter using a combustible composition, has been developed for greatly improving the precision of firing time. Electronic delay igniters, such as those disclosed in U.S. Pat. No. 4,445,435, U.S. Pat. No. 4,586,437, U.S. Pat. No. 4,712,477, Japanese Patent Application Publication No. 53479/1988. Japanese Patent Application Laid Open No. 111989/1986, Japanese Patent Application Laid Open No. 16582/1992, Japanese Patent Application Laid Open No. 79797/1993, are known.
These electronic delay detonators are divided into an analog type and a digital type depending on the delay means of the electronic timer unit, and the following three types are known.
The first is an analog type electronic timer using a CR circuit disclosed in U.S. Pat. No. 4,712,477. FIG. 1 is a block diagram of an electronic delay igniter using a CR circuit. As shown in the Figure, in this example, a resistor 1 and a capacitor 2 form a time constant circuit 3. The time constant circuit 3 is connected with a comparator circuit 4 for comparing a voltage stored in the capacitor 2 with a predetermined voltage, which detects a time at which the voltage stored in the capacitor 2 is the predetermined voltage. That is, the analog electronic timer uses the predetermined time when energy is supplied from a blasting machine (not shown) until the predetermined voltage is stored in the capacitor 2 as a delay time to output an output pulse after the lapse of a predetermined delay time. On the other hand, a circuit having an input resistor 5, a rectifier 6, and a voltage dividing resistors 7 and 8 is formed in a signal input unit. Firing energy is temporarily stored in a firing capacitor 9 through a rectifier 6, and this energy is supplied to an ignition unit through a switch circuit released by the output pulse output from the electronic timer after the delay time. Here, the switch circuit comprises switches 10 and 11, a latch 12, and a switch 13, and the ignition unit comprises a heater 14, and an ignition charge 15 which is in contact with the heater 14. Delay time of the electronic timer can be arbitrarily set by adjusting the resistance of the resistor 1 or the capacitance of the capacitor 2.
The second is a digital type electronic timer using a CR pulse oscillator disclosed in U.S. Pat. No. 4,586,437, and FIG. 2 is a block diagram of an electronic delay igniter using a CR pulse oscillator. As shown in the Figure, delay means of the electronic timer comprises an electronic timer circuit 21, a capacitor 22 and a resistor 23 which are connected to the electronic timer circuit 21, in which repeated charge and discharge of the capacitor 22 is made by a combination of the capacitor 22 and the resistor 23, and pulses having a generated predetermined frequency are counted by a counter circuit incorporated in an electronic timer circuit to output an output pulse. A signal input unit for a signal from the blasting machine is provided with a rectifier 24, a firing capacitor 25, and a constant voltage circuit 26. Further, firing energy temporarily stored in the firing capacitor 25 is supplied to an ignition unit comprising a heater 28 and an ignition charge 29 through a switching unit 27 which is released by the output pulse output from the electronic timer circuit after the lapse of the delay time.
The third is a digital type electronic timer using a solid state oscillator such as a quartz oscillator, which is disclosed in U.S. Pat. No. 4,445,435, Japanese Patent Application Publication No. 53479/1988, Japanese Patent Application Laid Open No. 11198/1986, Japanese Patent Application Laid Open No. 16582/1992, Japanese Application Laid Open No. 79797/1993.
The operation sequence of the above-described first to third electronic delay electric detonators is almost the same. Specifically, when a certain amount of energy is supplied from the blasting machine to the firing capacitor, the electronic timer begins to operate and, after the lapse of a predetermined time, an output pulse signal is transmitted from the electronic timer unit (or a blasting machine) to the switching unit. On receiving the signal the switching unit is released, and the electric energy stored in the firing capacitor is supplied to the ignition unit. The ignition unit comprises a heater and an ignition charge contacting the heater. When the electric energy stored in the firing capacitor is supplied, the heater is heated and, when the heater surface temperature reaches the ignition temperature of the ignition charge, the ignition charge ignites, thereby supplying heat energy to the initiating unit. Thus, the electronic delay electric detonator is initiated.
Here, the time precision of the delay means of the first and second electronic delay electric detonators, when viewed from only the electronic delay unit, depends upon the CR circuit using CR. Since, in such a CR pulse oscillator circuit, the time precision is basically determined by the device characteristics of the capacitor C and the resistor R of the time constant circuit. For determining the time, a capacitance deviation or the like of the device must be allowed. For example, the time precision is.+-.several .mu.s to over 10 .mu.s for a reference time of 1000 ms.
On the other hand, the third electronic delay electric detonator uses a solid state oscillator. In this case, since the solid state oscillator, itself, is high in oscillation precision, a time precision of.+-.several tens of .mu.s to several hundred .mu.s can be obtained for a reference time of 1 second.
Considering the fact that prior art electric detonators using a combustible composition have a large deviation of 5 to 10% based on the reference time, these electronic delay electric detonators having delay means are sufficiently distinct when compared with such prior art electric detonators.
As described above, in the electronic delay electric detonator, operation of the electronic timer and other circuits and ignition of the ignition unit are carried out with only the electric energy stored in the firing capacitor. Therefore, it is preferable to use a capacitor having a capacity as large as possible, and to be charged to as high a voltage as possible in order to increase the charge amount, but in a practical design, an appropriate capacity must be selected so that the size is not too large. Further, even for firing multiple detonators, the charge voltage of the firing capacitor is required to be suppressed to about 25 V so that the firing voltage and the capacitance of blasting machine are not excessive. Therefore, the consumption current in the electronic timer and the firing energy in the ignition unit are normally suppressed as much as possible.
For an electric detonator, energy required for firing the ignition unit (minimum firing energy) includes several grades in terms of external electric hazard factors, such as stray current and leakage current. Normally a type of small energy of about 2 to 4 mJ is used.
On the other hand, such an igniter is naturally required to have a high initiation reliability. Normally, for an igniter such as an electric detonator, it is a legal obligation to perform a continuity test immediately prior to firing to check the firing circuit against abnormality, and it is particularly important for ignition reliability to check the continuity (resistance) of the firing circuit at the final step in the production process.
Naturally, for an electronic delay igniter, the firing circuit is also required to be checked as the final step of production in view of the ignition reliability. For the electronic delay igniter, in view of the nature of the circuit, it is required to operate the switching circuit in order to check the firing circuit. As a circuit check device, the inventors have developed a continuity checker for electronic delay electric detonator (Japanese Patent Application Laid Open No. 99597/1993).
Whether it is an electric detonator or an electronic delay electric detonator, checking the igniter must be carried out in the state provided with the ignition charge. Checking the firing circuit of the electric detonator is sufficient only by a continuity check. Since it is carried out using a small current of normally 10 mA, there is less danger of heating the heater to induce explosion. However, an electronic delay electric detonator has a difficult problem described below because the firing circuit mechanism differs from that of the prior art electric detonator.
When checking the firing circuit of the electronic delay electric detonator, it is necessary to operate the electronic timer for a predetermined period of time to obtain an output signal, and make sure that the switching unit operates. For this purpose, the firing capacitor is required to be subjected to a voltage higher than the operating voltage. Therefore, since the current in the ignition unit varies depending on the capacity of the capacitor, the voltage, and the heater resistance, and the like, in some cases, after the switching operation a substantial current may flow, leading to spontaneous explosion.
On the other hand, with recent advances in firing techniques, when blasting is attempted to be controlled by the initiation time, merely improving the time precision considerably in comparison to the prior art electric detonator is sufficient, in that a precision of .+-.0.5 ms is required, as will be described below.
In blasting, for example, the following estimation formula corresponds to a theory that an optimum initiation time difference is the time for explosion gas pressure generated by the explosive, to interact with the adjacent bore hole.
DT=L.times.1000/(V.times.0.12) PA1 DT: optimum initiation time difference (ms) PA1 L: hole interval (m) PA1 V: elastic wave velocity in a breast site rock (m/s)
That is, it is said that the best blasting effect can be obtained when initiating the next hole under the action of explosion gas. Then, using the estimation formula, optimum initiation times for a light place and a place in tunnel are determined as follows.
For the light place, the hole interval is 3-5 m, and the calculation is as follows. ##EQU1## wherein V (limestone)=2000 to 30000 m/s.
For a place in tunnel, the hole interval is less than 1 m, and the calculation is as follows. ##EQU2## wherein V (medium hard rock)=4000 to 5000 m/s.
Therefore, with deviations according to the site conditions, in general, a time interval of 8 to 20 ms is optimum for a light place, and error must be less than .+-.2 ms when an allowance of .+-.10% is given. Further, for a place in tunnel where the hole interval is small, in particular for blasting hard rock, deviation must be less than .+-.0.5 ms in absolute precision.
Thus, in an electronic delay electric detonator with the aim of blasting control, an absolute precision of .+-.0.5 ms is required.
Therefore, in this case, the use of a digital type electronic timer having a solid state oscillator is essential as delay means. However, the use only of a digital timer is not always sufficient, to achieve high precision firing. For practically viable values of the capacity of the firing capacitor, the voltage, and the like, selection of the ignition charge is extremely important.